This invention relates to an electro-optic phase retardation device. More particularly, it relates to such a device in a rigidly bonded sandwich-like configuration and exhibiting high optical response speed in the switching of a transparent ferroelectric ceramic element between birefringent and substantially isotropic states.
Electro-optic systems and devices are well known and are described, for example, in U.S. Pat. Nos. 3,069,973 (issued Dec. 25, 1962 to I. Ames); 3,467,463 (issued Sept. 16, 1969 to N. F. Borrelli et al); 3,512,864 (issued May 19, 1970 to G. H. Haertling et al); 3,713,723 (issued Jan. 30, 1973 to W. R. Buchan); and 4,057,324 (issued Nov. 8, 1977 to Kawashima et al). Electro-optic devices utilize variations in an electric field applied across a transparent active element to produce variations in the refractive index of the element in order to modulate the intensity of polarized light passing through the device. Such devices will normally comprise a controllable birefringent element located between two polarizers having their planes of polarization at right angles to one another. The light input passes through the first polarizer and enters the element as plane-polarized light. In the absence of an electrical field across the active element, the plane-polarized light passes through the element without rotation and is totally absorbed by the second polarizer. However, when an electrical field is applied to the element, the element becomes birefringent, and the light emerges from the element as elliptically or circularly polarized light, part of which passes through the second polarizer, or analyzer.
Electro-optic ceramic materials of the quaternary (Pb.sub.1-x La.sub.x) (Zr.sub.y Ti.sub.z).sub.1-x/4 O.sub.3, system, commonly known as PLZT, have been intensively investigated in recent years. These ceramic materials are polycrystalline lanthanum-modified lead zirconate-titanate materials which exhibit uniaxial birefringence on a macroscopic scale when polarized by an external field and their light transmission characteristics are similar to those of uniaxial crystals. The high optical transmission properties of the PLZT materials and the availability of such materials having a range of desirable compositional and electro-optic properties have led to utilization of the PLZT ceramics in a number of electro-optic applications. PLZT materials, their properties, methods of preparation and applications thereof can be found, for example, in U.S. Pat. No. 3,666,666 (issued May 30, 1972 to G. H. Haertling) and in the publications "Hot-Pressed Ferroelectric Lead Zirconate Titanate Ceramics for Electro-Optical Applications" (G. H. Haertling, The American Ceramic Society Bulletin, Vol. 49 No. 6, June 7, 1970, pp. 564-567) and "Improved Hot-Pressed Electrooptic Ceramics" in the (Pb, La) (Zr, Ti)O.sub.3 System, Journal of the American Ceramic Society, Vol. 54 No. 6, June, 1971, pp. 303-309).
The utilization of PLZT ceramic elements in electro-optic devices of the fast-shutter type has been described by J. T. Cutchen et al, in "Electrooptic Devices Utilizing Quadratic PLZT Ceramic Elements", a paper presented at the 1973 Western Electronic Show and Convention (Wescon), Session 30, on Ferroelectric Ceramic Electro-Optic Devices, Sept. 13, 1973. The employment of PLZT elements in electro-optic devices of such type, particularly in connection with flashblindness protective devices in goggles, glasses, helmets, and the like, has been contemplated. In this connection, the utilization of a PLZT ceramic wafer pressed between two crossed polarizers in a sandwich-like configuration has been proposed. The application of an electric field to the PLZT ceramic element (the "ON" state) renders the ceramic element birefringent, the birefringence being a quadratic dependence on the electric field (known as the Kerr effect). In the field-induced "ON" state, the electro-optic device is light transmissive. At zero field (the "OFF" state), the PLZT wafer exhibits substantially no birefringence, thus, rendering the electro-optic device non-transmissive to light, for example, optical density four to five. It will be appreciated that in an electro-optic device for the prevention of flashblindness, a closure from the relatively high-transmissive mode to the protective high-density mode should be as rapid as possible.
The application of an electric field to a PLZT wafer induces mechanical strains in the ferroelectric element as the result of ferroelectric polarization of grains of the ceramic element. While a rapid discharge of the electric field, as by discharge through a resistance in response to a signal from a photoconductor cell or other light-responsive element, will effect a relaxation of these strains, the return of the ceramic element to optical isotropy is not instantaneous and oscillations in the level of light transmission accompany oscillations in the PLZT ceramic element. The result is that transmission of light decreases with time until the desired protective optical density of the fully-"OFF" state is achieved.
While the mechanical clamping of rigid isotropic elements to a PLZT ceramic wafer will permit sufficient mechanical integrity for an electro-optic device of the fast shutter type, and will enable the device to be rapidly switched from the "ON" state to a protective "OFF" optical density, the mechanical coupling of the elements of such a device is particularly incompatible with the objective of providing an integral and easily handled electro-optic device capable of utilization in such devices as goggles, helmets or the like without misalignment or separation of the elements. In addition, air gaps between mechanically clamped elements give rise to reflection losses. The utilization of an adhesive material to bind the PLZT ceramic element to rigid elements, such as glass, and to provide structural integrity and compactness, is frequently observed to result in deterioration of the switching time or optical response speed (ORS) of the PLZT element.
It is an object of the present invention to provide a rigidly bonded electro-optic device.
It is another object of the present invention to provide a preassembled and rigidly bonded electro-optic device having good mechanical coupling and integrity of the elements thereof and exhibiting rapid optical switching characteristics.
A further object is to provide such a device capable of transmitting light in a field-induced state and rapidly switchable to a protective "OFF" state optical density.
Still another object of the invention is to provide such a device capable of utilization in flashblindness protection devices.
Other objects will become apparent from the description appearing hereinafter.